Fatigue Life of ProTaper GoldRM Instrumentsrepositorio.ul.pt/bitstream/10451/25512/1/ulfmd02852_tm_Fatima... · de mestrado, marca o fim de uma etapa. Àqueles que fizeram parte deste

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Universidade de Lisboa

Faculdade de Medicina Dentária

Fatigue Life of ProTaper GoldRM

Instruments

– An in vitro study –

Fátima Carolina Ferreira Pereira

Dissertação

Mestrado Integrado em Medicina Dentária

2015

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Universidade de Lisboa

Faculdade de Medicina Dentária


Instruments

– An in vitro study –

Fátima Carolina Ferreira Pereira

Dissertação orientada pelo Prof. Doutor António Ginjeira e

co-orientada pelo Prof. Doutor Rui Martins (FCT-UNL/DEMI)

Dissertação

Mestrado Integrado em Medicina Dentária

2015

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“Reconhecer a verdade como verdade, e ao mesmo tempo como erro; viver os

contrários, não os aceitando; sentir tudo de todas as maneiras, e não ser nada, no fim,

senão o entendimento de tudo.”

Fernando Pessoa

http://pensador.uol.com.br/autor/fernando_pessoa/

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AGRADECIMENTOS

Por vezes o mais difícil não é fazermos tudo sozinhos, mas sim pedir ajuda e

conseguir expressar a devida gratidão quando a recebemos. Assim, gostaria de

agradecer àqueles que ajudaram-me a construir este documento que mais que uma tese

de mestrado, marca o fim de uma etapa. Àqueles que fizeram parte deste percurso de 5

anos fantásticos. E, acima de tudo, àqueles que me acompanham há uma vida e que

espero que sempre façam parte dela.

Ao Prof. Doutor António Ginjeira pela orientação que me ofereceu neste

trabalho assim como por ser uma das pessoas mais acessíveis, ensinando-me sempre

como aluna e como pessoa.

Ao Prof. Doutor Rui F. Martins pela sua paciência, gentileza, método e

disponibilidade imensa.

Ao Prof. Henrique Luís, pela ajuda indispensável que prestou na área da análise

estatística.

À Patrícia Quaresma por partilhar este percurso comigo, pois “…a vida são 2

dias e a tese ficou para Setembro...”.

Aos meus colegas e amigos de faculdade que tornaram este percurso tão mais

leve e cheio de experiências, em especial à Carolina, Filipa, Luana, Fábia, Adriana,

Edgar, João e Marialice.

Aos meus amigos de e para sempre João Tiago, Margarida, Leonor, Ana, Nuno,

Francisco; especialmente à Mariana pela amizade constante ao longo dos anos.

E, acima de tudo, à minha família, em especial aos meus tios Zélia e Gregório e

primos Maria e Tomás. Sem eles, estou certa que não seria quem sou, o que sou e onde

sou. Em especial à minha tia que representou muita coisa numa pessoa só, substituindo

muito do que no resto tive em falta.


Instruments – An in vitro study

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Fátima Pereira i FMDUL

RESUMO

Introdução: Endodontia é o ramo da Medicina Dentária que abrange o estudo,

em particular, da polpa dentária assim como da raiz e dos tecidos periapicais. O

tratamento de canais tem por objetivo preservar o dente, funcionalmente, facilitando a

resolução da inflamação canalar e periapical. Este tipo de tratamento compreende vários

passos, de componente química e mecânica, que envolvem a remoção do tecido pulpar e

desinfeção do sistema canalar. A preparação mecânica, por sua vez, e através da

utilização de vários instrumentos (manuais ou mecânicos) promove a desinfeção e

shaping dos canais radiculares.

No entanto, a presença de uma anatomia canalar complexa e limitações inerentes

aos próprios instrumentos utilizados no tratamento podem revelar-se um desafio. Desta

forma, vários sistemas de instrumentos foram desenvolvidos, com diferentes

propriedades mecânicas que os distinguem.

Recentemente, instrumentos fabricados a partir de ligas de Níquel-Titânio (NiTi)

com resistência à fadiga consideravelmente superior e com design inovador, permitem

um alargamento de canais radiculares curvos de forma mais eficaz e segura,

preservando a anatomia.

Uma vez que a popularidade destes instrumentos é crescente, uma maior

preocupação com a possibilidade de fratura durante a instrumentação está latente. Dois

mecanismos de fratura estão largamente descritos na literatura: a fratura por fadiga e a

fratura por torção. A fratura por fadiga ocorre quando um instrumento gira livremente

num canal, gerando ciclos de tensão/compressão no ponto de máxima curvatura, até que

a fratura ocorra. Esta, parece ser a maior causa de fratura nos instrumentos rotatórios

endodônticos e pode ser avaliada através do Número de Ciclos à Fratura (NCF), dado

pelo número de ciclos necessários até à fratura do instrumento.

Diversos fatores como o tipo de liga metálica, tratamento de superfície, secção

de corte e dimensões do instrumento afetam a flexibilidade e NCF de diferentes limas.

Deste modo, várias estratégias têm sido utilizadas de forma a aumentar a sua resistência,

no culminar de diferentes sistemas de instrumentos rotativos.

Um dos sistemas de instrumentos NiTi rotatórios mais descrito é o sistema

ProTaper® Universal (PTU) (Dentsply Maillefer, CH). Com uma conicidade progressiva

ao longo do seu comprimento, secção triangular, centro de rotação coincidente com o

centro de massa e ponta inativa, a sequência básica de instrumentação compreende a



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Fátima Pereira ii FMDUL

utilização de 6 instrumentos: 3 para preparar o terço médio e coronal (SX, S1 e S2) e

outros 3 para alargar o terço apical do canal (F1, F2 e F3).

O sistema ProTaper NextTM

(PTN) (Dentsply Tulsa Dental, OK, USA) foi

lançado em Abril de 2013, com algumas características diferenciadoras, das quais:

percentagem de conicidade progressiva, tecnologia M-Wire® e configuração off-set.

Com secção rectângular e centro de massa não coincidente com o centro de rotação, este

sistema é composto por 5 instrumentos: X1, X2, X3, X4 e X5.

Recentemente foi introduzido no mercado o sistema ProTaper GoldRM

(PTG)

(Dentsply Tulsa Dental Specialities). A sua configuração geométrica compreende os

mesmos princípios que o sistema PTU, assim como o mesmo número, tipo de

instrumento e indicações de uso. No entanto, um tratamento de superfície com

tecnologia CM-Wire® parece diferenciar este sistema, além de um comprimento de cabo

2 milímetros menor, que promete uma acessibilidade mais fácil ao dente. No entanto, a

relação entre o tratamento de superfície e as propriedades de fadiga deste sistema têm

pouca pesquisa independente disponível.

Materiais e Métodos: Foram analisadas 48 limas endodônticas, novas e sem

utilização prévia, de 25 mm, do sistema PTG e PTU. 4 Grupos experimentais foram

formados, de acordo com o tipo de sistema e lima utilizados – PTG F2 (n=12), PTG F3

(n=13), PTU F2 (n=12) ou PTU (n=12).

No seguimento dos estudos que têm vindo a ser desenvolvidos no âmbito da

colaboração estabelecida entre o Departamento de Endodontia da Faculdade de

Medicina Dentária da Universidade de Lisboa e o Departamento de Engenharia

Mecânica da Faculdade de Ciências e Tecnologias da Universidade Nova de Lisboa, foi

criado um sistema mecânico em que os instrumentos foram submetidos a forças que

mimetizam um canal radicular. O raio de curvatura estabelecido foi de 4,7 mm e o

ângulo de curvatura de 45˚. Cada instrumento foi inserido no contra-ângulo acoplado ao

micromotor WaveOneTM

e submetido ao teste de fadiga com uma velocidade de rotação

de 300 rpm e um binário de 4 N. cm. O tempo que a lima demorou a fraturar, foi

registado com um cronómetro digital, sempre pelo mesmo operador. De seguida, o NCF

foi calculado pela multiplicação da velocidade de rotação pelo tempo decorrido até à

fratura.

A nível da análise estatística, os dados obtidos em relação ao tempo e NCF

foram analisados pelo teste paramétrico de variáveis independentes de t-student, uma



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Fátima Pereira iii FMDUL

vez que a amostra apresentou uma distribuição normal. Por outro lado, os dados

relacionados com o comprimento de fratura foram estatisticamente analisados pelo teste

não paramétrico de U Mann-Whitney, uma vez que a amostra não apresentava

normalidade.

De forma a comparar a média de tempo de fratura com os dados relativos ao

estudo de Vaz 2014, um estudo in vitro que analisa o sistema PTN, o mesmo

procedimento foi utilizado.

Resultados: Considerando as hipóteses, conclui-se: a média de NCF entre o

grupo 1 – PTG F2 e o grupo 2 – PTG F3 é estatisticamente superior para o grupo 1 com

um valor de p <0,001. Além disso, o valor de NCF para o grupo 3 – PTU F2 em relação

ao grupo 4 – PTU F3 foi estatisticamente superior (p <0,001).

O local de fratura não mostrou, estatisticamente, ser significativamente diferente

de acordo com o tipo de instrumento, entre os grupos 1 e 2, 1 e 3 e ainda entre os grupos

3 e 4. O mesmo não se verifica entre os grupos 2 e 4, sendo o comprimento de fratura

estatisticamente superior para o grupo 4 (p <0,014).

O tempo até a fratura ocorrer entre os grupos 1 e 3, foi estatisticamente superior

para o grupo 1, com valor p <0,001. O mesmo ocorre para o grupo 2 quando comparado

com o grupo 4 (p <0,001).

Quando os dados entre as PTG e as PTN foram comparados, foi notado um

tempo para a fratura estatisticamente superior para o grupo 1, em relação ao presente na

amostra de PTN X2 (p <0,001). No entanto, o tempo até a fratura da amostra de PTN

X3 foi estatisticamente superior à do grupo 2.

Discussão e Conclusões: Os instrumentos PTG F2 provaram ser

significativamente mais resistente à fadiga que os instrumentos PTG F3. O mesmo pode

ser afirmado para a amostra de PTU F2 em relação com a amostra de PTU F3. Estes

resultados parecem estar relacionados com o aumento no diâmetro que se verifica entre

os instrumentos F2 e F3.

Além disso, as amostras de PTG F2 e PTG F3 provaram ser estatisticamente

superiores, no que toca ao tempo para a fratura, que as PTU F2 e PTU F3,

respetivamente. Esta análise está de acordo com o observado em estudos prévios e a

razão parece estar relacionada com a tecnologia CM-Wire® utilizada no fabrico dos

instrumentos PTG.



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Fátima Pereira iv FMDUL

Por outro lado, a média para o comprimento de fratura entre as PTG F2 e as

PTG F3 e para as PTG F2 em relação com as PTU F2 não foi significativa. É referido

na literatura que a fratura ocorre normalmente no ponto de máxima curvatura, no

entanto houve uma diferença estatisticamente significativa entre os instrumentos PTG

F3 e PTU F3. Esta diferença pode dever-se a vieses inerentes ao próprio estudo, sendo a

tomada de conclusões complexa.

Dados referentes ao estudo feito por Vaz 2014, que analisou 24 limas do sistema

PTN sob o mesmo sistema mecânico e procedimento que o presente estudo, revelaram

que o tempo médio para a fratura das PTG F2 foi superior ao observado para as PTN

X2, e que as PTN X2, por sua vez, têm um valor médio superior às PTU F2. Isto pode

dever-se a diferentes composições metálicas entre os instrumentos, para além de

diferenças a nível de secção de corte e tratamento de superfície. Por um lado,

instrumentos construídos com CM-Wire®

têm uma maior resistência à fadiga em relação

a outros instrumentos, segundo estudos recentemente desenvolvidos. Por outro, foi

demonstrado que uma secção triangular é compatível com melhores resultados de

resistência à fadiga entre instrumentos.

Além dos resultados obtidos através da análise estatística das amostras,

comparam-se os dados com estudos referentes ao mesmo sistema, onde denotaram-se

valores para a resistência à fadiga muito variáveis. Estas variações parecem estar

relacionadas com diferenças a nível do ângulo e raio de curvatura dos canais utilizados,

assim como diferenças a nível ambiente experimental.

Posto isto, conclui-se que o sistema ProTaper GoldRM

parece apresentar uma

resistência à fadiga superior em relação a sistemas previamente desenvolvidos, sendo

um sistema a inserir no leque de opções válidas para utilização clínica. Além disso, há

urgência em criarem-se padrões específicos e internacionais para testar a resistência à

fadiga de instrumentos rotatórios. O mesmo pode ser referido no que toca ao método

mais preciso para a análise estatística correspondente, de forma a atingir uma

comparação de evidência científica superior.

Palavras-chave: ProTaper Gold; Resistência à Fadiga, Instrumentos Níquel-

Titânio; Instrumentação Mecanizada; Endodontia.



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ABSTRACK

Introduction: Nickel-titanium instruments were introduced to facilitate canal

preparation. Despite its advantages, instrument separation remains a major concern in

endodontics due to several factors. There are many systems of endodontic files and the

purpose of this study was to characterize the cyclic fatigue of ProTaper GoldRM

(PTG)

instruments and compare it to Protaper® Universal (PTU) and other rotary systems.

Materials and Methods: Forty-eight rotary nickel-titanium files of PTG and

PTU systems were analyzed in this study. Those were divided into four experimental

groups, according to type of system and instrument (PTG F2, PTG F2, PTG F3, PTU F2

and PTU F3). A mechanical device was used to simulate the root canal system with a

radius of curvature of 4, 7 mm and an angle of curvature of 45˚. Each instrument was

submitted to testing with rotational speed of 300 rpm and a torque of 4 N. cm. Testing

time was registered when tip separation occurred. Data obtained such as time to fracture

of the instrument tested and number of cycles to fracture (NCF) was statically analyzed

by t-student test. For fracture length the non-parametric U-Whitman test was used. PTG

and ProTaper NextTM

(PTN) data from Vaz 2014 study were analyzed with the same

tests. Significance was set at 95% confidence level.

Results: PTG F2 instrument proved to be statistically more resistant to cyclic

fatigue than instruments PTG F3, PTU F2 and PTN X2.

Discussion and Conclusions: Compared with different rotary systems such as

PTU and PTN, this system suggests being more resistant to cyclic fatigue. During

clinical practice, clinicians should be aware of the mechanical properties of the

instruments chosen and take into account the higher resistance to cyclic fatigue of

ProTaper GoldRM

files when compared to other systems.

Key-words: ProTaper Gold; Cyclic Fatigue; Nickel-titanium Instruments;

Rotary Preparation; Endodontics.



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INDEX

RESUMO .......................................................................................................................... i

ABSTRACK ..................................................................................................................... v

I. Introduction ................................................................................................................. 1

A.Type of instruments ............................................................................................. 1

B.Instrument composition ....................................................................................... 2

C.When fracture occurs ........................................................................................... 4

D.Improving performance ....................................................................................... 5

II. Aims .......................................................................................................................... 10

III. Materials and Methods .......................................................................................... 12

A.Instruments ........................................................................................................ 12

B.Equipment ......................................................................................................... 13

C.Experimental procedure..................................................................................... 15

D.Statistical analysis ............................................................................................. 16

IV. Results .................................................................................................................... 18

V. Discussion ............................................................................................................... 23

VI. Conclusion .............................................................................................................. 29

REFERENCES ................................................................................................................ ix

APPENDIX ................................................................................................................... xiii



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FIGURES INDEX

Figure 1. Diagramatic representation of the martensitic transformation and shape

memory effect of NiTi alloy (Thompson 2000). .............................................................. 3

Figure 2. Pruett’s method for describing canal geometry using two parameters: radius

of curvature and angle of curvature (Pruett et al. 1997) ................................................... 5

Figure 3. Rate of taper in ProTaper® Universal file (Dentsply Maillefer) ...................... 6

Figure 4. ProTaper® Universal system composed by shaping and finishing files

(Dentsply Maillefer) ......................................................................................................... 7

Figure 5. ProTaper® Universal F3, F4 and F5 finishing files (Dentsply Maillefer) ........ 7

Figure 6. ProTaper NextTM

rectangular cross section (Dentsply Tulsa Dental) .............. 8

Figure 7. ProTaper NextTM

system composed by X1, X2, X3, X4 and X5 (Dentsply

Tulsa Dental). ................................................................................................................... 8

Figure 8. ProTaper GoldRM

system composed by shaping and finishing files (Dentsply

Tulsa Dental Specialties). ................................................................................................. 9

Figure 9. Samples used for the in vitro study ................................................................ 12

Figure 10. Test bench with general measures and prototype (Fernandes 2013) ........... 13

Figure 11. Schematic representation of the mechanical system adapted from Pinto 2013

........................................................................................................................................ 14

Figure 12. Mechanical system, ProTaper GoldRM

and ProTaper®

Universal during ciclic

fatigue testing. ................................................................................................................ 14

Figure 13. Support structure with extended holes. ........................................................ 15



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TABLES INDEX

Table 1. Samples distributed through the experimental groups with respective length

(mm) and taper................................................................................................................ 12

Table 2. Time to fracture, fracture length and NCF data from group 1 and 2 – ProTaper

GoldRM

instruments. ....................................................................................................... 18

Table 3. Time to fracture, fracture length and NCF data from group 3 and 4 –

ProTaper® Universal instruments. .................................................................................. 19

Table 4. Descriptive analys: mean, standard deviation and variance regarding time,

length of fracture and NCF. ............................................................................................ 20

Table 5. Mean time to fracture data from the present study and from Vaz 2014 study. 21

Table 6. Summary conditions, design and results of 2 studies made for ProTaper

GoldRM

cyclic fatigue testing. ......................................................................................... 26

CHARTS INDEX

Chart 1. Graphical representation of NCF for groups 1, 2, 3 and 4. ............................. 20

Chart 2. Mean time to fracture (sec) for group 1 – PTG F2, PTN F2 and group 3 – PTU

F2. ................................................................................................................................... 22



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I. Introduction

Endodontics is the branch of dentistry, concerned with morphology, physiology,

and pathology of the human tooth, and in particular the dental pulp, root and peri-

radicular tissues (Wu & Wesselink 1993).

Pulp disease is usually inflammation of connective tissue (the pulp), which can

be caused by any type of injury (mechanical, physical, chemical, thermal or electrical)

(European Society of Endodontology 2006).

Treatment aimed at preserving a functional pulp by facilitating resolution of pulp

inflammation is called vital pulp therapy. Treatment aimed at preserving a functional

tooth by facilitating resolution of periapical inflammation is called root canal treatment

(Gulabilava & Ng 2014).

Root canal treatment involves the removal of pulpal tissue and the disinfection

of the root canal system, by series of steps (mechanical and chemical). Mechanical

preparation using a variety of instruments (both manual and machine driven) promotes

cleaning and shaping, in which the ideal prepared root canal shape is a three-

dimensional continuously tapering cone, narrowest apically and widest at the root canal

entrance, always respecting the root anatomy.

However, some challenges can be found during root canal preparation, like

complex anatomy of the root canal system and instrumental limitations inherent to

treatment (Gulabilava & Ng 2014; Patel & Barne 2013).

A. Type of Instruments

Endodontic instruments for root canal preparation can be divided into three

groups:

1. Hand-and finger-operated instruments.

2. Low-speed instruments.

3. Engine-driven instruments.

Historically most instruments used were designed to be used by hand. Although

not universally used, rotary instrumentation has gained considerable interest and can

enhance the quality of treatment (Cohen & Hargreaves 2006).



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B. Instrument composition

1. Stainless Steal

At first, root canal instruments were manufactured from carbon steel. However,

chemicals (e.g. iodine, chorine) and steam sterilization caused significant corrosion.

Subsequently, the use of stainless steel greatly improved the quality of instruments (Wu

& Wesselink 1993).

Still, the high stiffness value of typical steel – the property of a solid body to

resist deformation – remained as the great disadvantage of these instruments. This

feature produced forces in the anti-curvature wall causing wear and modification of the

original root canal shape during instrumentation. Consequently the prognosis could be

worst due to non-successful instrumentation (Plotino et al. 2009).

2. Nickel-Titanium

More recently, Nickel-Titanium (NiTi) alloys represented a major breakthrough

in Endodontics, overcoming the high stiffness of instruments made of stainless steel.

With considerably higher fatigue resistance than stainless steel instruments of

similar size, together with innovative file designs, a more effective and safer

enlargement of curved canals, without significantly losing their original path, is possible

and at a much faster rate than hand files. (Tripi et al. 2006; Lopes et al. 2009; Lee et al.

2011)

- NiTi structure

The NiTi alloys used in endodontic instruments are generically called 55-

Nitinol, which contain approximately 56% nickel and 44% titanium. The resultant

combination is a one-to-one atomic ratio (equiatomic) of the major components.

The crystalline structure of NiTi alloy at high temperature ranges (≥100˚C) is a

stable, face-centered cubic lattice which is referred to as the Austenite Phase or Parent

Phase (Fig.1).



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Figure 1 - Diagramatic representation of the martensitic transformation and shape

memory effect of NiTi alloy (Thompson 2000).

When it is cooled through a critical transformation temperature range (TTR),

there is a change in the crystal structure which is known as the martensitic

transformation that originates the so called Martensitic or Daughter Phase (Fig 1).

The martensite shape can be deformed easily to a single orientation by process known

as de-twinning.

The transition from the austenitic to martensitic phase can also occur as a result

of application of stress, such occurs during root canal preparation. This stress-induced

martensitic (SIM) transformation is reversible; hence the material exhibits an unusually

large elastic range and is able to recover from a much higher strain than stainless steel

can withstand without breaking (Hou et al. 2010; Shen et al. 2011; Tsujimoto et al.

2014).

It is the crystalline change phenomenon described earlier which gives rise to the

shape memory effect of the material and the superelastic behavior (Kauffman &

Mayo 1997; Thompson 2000; Stojanac et al. 2012).

Hereupon, during the last decade NiTi rotary instruments have been gaining

popularity among almost all dentists practicing endodontic therapy. However, visible

inspections is not a reliable method for the evaluation of the physical integrity of NiTi

instruments, therefore an increasing concern about instrument fracture during clinical

procedure is present (Lopes et al. 2009; Lee et al. 2011).



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C. When fracture occurs

Two distinct fracture mechanisms have been identified, namely due to: fatigue

crack propagation (cyclic fatigue fracture) and due to torsional failure.

- Torsional failure occurs when an instrument tip or another part of the

instrument is locked in a canal while the shank continues to rotate.

- Cyclic fatigue fracture, in which the instrument rotates freely inside a

curvature canal, generating tension/compression cycles at the point of maximum

bending until fracture occurs. This type of fracture is due to metal fatigue and is usually

localized at the point of maximum curvature (Li et al. 2002; Plotino et al. 2009; Wan et

al. 2011; Lee et al. 2011; Lopes et al. 2011; Bouska et al. 2012).

According to Cheung et al. the great majority (93%) of instruments appeared to

have failed due to cyclic fatigue. This might be explained as follows: fatigue-crack

growth rates in NiTi alloys have been reported to be significantly greater than in other

metals of similar strength. Thus, once a micro-crack is initiated, it can quickly propagate

to cause catastrophic failure (Stojanac et al. 2012).

Therefore, understanding cyclic fatigue resistance of different endodontic

instruments may be a subject of interest and different tests for cyclic fatigue emerged.

1. Cyclic Fatigue Testing

The fatigue life of a material is the number of fatigue cycles required to its

failure. The cyclic fatigue tests are a simple and reliable approach to determine the

fatigue behavior of instruments manufactured from the NiTi alloy (Tripi et al. 2006).

Since the number of cycles until failure is cumulative, it can be obtained through

the multiplication of the rotation speed by the time elapsed until fracture occurs (Lopes

et al. 2009).

The devices used to determine the fatigue resistance of endodontic instruments,

allow instruments to rotate until fracture using different geometric curvatures (Plotino et

al. 2009).

Rotational bending fatigue tests can also be carried out with or without the axial

movement of the endodontic instrument. In static tests, the instrument rotates, with no



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axial displacement, whereas in the dynamic model the instrument is moved back and

forth within the canal (Rodrigues et al. 2011).

Earlier cyclic fatigue studies have noticed the influence of canal shape on

instruments breakage. Canal curvature can be expressed by the radius of curvature and

the angle of curvature as seen in Figure 2 (Pruett et al. 1997).

- The radius of curvature is the radius of the circle that approaches the

curvature of the canal most tightly – the radius of the osculating circle.

- The angle of curvature is the angle between two radii of the osculating

circle intersecting the end points of the canal curvature.

Figure 2 - Pruett’s method for describing canal geometry using two parameters: radius

of curvature and angle of curvature (Pruett et al. 1997).

The variation of both the angle and the radius of curvature of a canal will induce

different stresses on an instrument, thereby extending or reducing its fatigue life (Wan

et al. 2011).

To date, there is no specification or international standard to define cyclic

fatigue resistance of endodontic rotary instruments. As a result, several devices and

methods have been used to investigate in vitro cyclic fatigue failure (Plotino et al.

2009).

D. Improving performance

Some factors, including the type of metal alloy, impurities, heat treatments,

number of threads, helical angle, cross-sectional shape and dimensions affect the



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flexibility and cyclic lifespan of files (Parashos et al. 2006; Gutman & Gao 2011; Lee et

al. 2011; Shen et al. 201;, Versluis et al. 2012; Uygun et al. 2015).

Hence, strategies have been used to improve the fatigue resistance of NiTi

endodontic instruments. Recently, thermal treatment of NiTi alloys, e.g. Controlled

Memory wire (CM-Wire®) (DS Dental, Johnson City, TN), Memory wire (M-Wire

®)

(Dentsply Tulsa Dental Speacialities, Tulsa, OK), and R-phase wire (SybronEndo,

Orange, CA) has been used. (Gutman & Gao, 2011; Condorelli et al. 2010; Shen et al.

2011; Pérez-Higueras et al. 2014; Hiewy et al. 2015; Capar et al. 2015).

That way, several NiTi file systems are currently available with differentiating

characteristics that may attribute clinical advantages (Tripi et al. 2006; Larsen et al.

2009).

1. ProTaper® Universal

ProTaper®

Universal (PTU) (Dentsply Maillefer, CH) is a well-described NiTi

rotary system of instruments manufactured with progressive taper over the length of the

cutting blades, triangular cross-sections, and non-cutting tips (Hiewy et al. 2015).

Figure 3 - Rate of taper: the rate of taper varies along the cutting flutes of each

ProTaper® Universal file (Dentsply Maillefer).

The basic sequence to shape root canals with PTU includes 6 instruments, 3 of

them to prepare the coronal and middle third (SX, S1 and S2) and the other 3 to enlarge

the apical third (F1,F2 and F3), (Fig. 4) (Pérez-Higueras et al. 2015).



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Figure 4 - Protaper® Universal system composed by shaping and finishing files

(Dentsply Maillefer).

Shaping files pre-enlarge and shape the coronal 2/3 of the canal with brushing

movements. Finishing files finish the apical 1/3 and only can be used until they reach

the full working length, no brushing movements. These files have, in sequence, purple

(S1), white (S2), yellow (F1), red (F2, blue (F3), double black (F4) and double yellow

(F5) identification rings corresponding to sizes 18/02, 20/04, 20/07, 25/08, 30/09, 40/06

and 50/05.

There’s also available SX shaper file, used only with the purpose of improving

canal access, size 19/04.

SX, S1, S2, F1 and F2 have a convex triangular cross section that is responsible

for giving them resistance. F3, F4 and F5, present a different section, with concave

triangular cross sections, giving them flexibility (Fig. 5).

Figure 5 – ProTaper® Universal F3, F4 and F5 finishing files feature a reduced cross section. A

convex, triangular cross section reduces contact with canal wall (Dentsply Maillefer).

These files are available in 3 lengths, 21, 25 and 31 mm, and have a rotation

center coincing with their mass center (Ruddle 2008; Hiewy et al. 2015; Dentsply

Maillefer).



2014/2015


2. Protaper NextTM

The Protaper NextTM

rotary file system (PTN) (Dentsply Tulsa Dental, OK,

USA) had its market debut on April 2013 and, according to the manufacturers, these

files are the convergence of three significant design features: progressive percentage

tapers on a single file, M-Wire® technology and off-set configuration.

The rectangular cross section along with the non-coincidence between the

rotation center and the mass center of the file, results in a limited contact of the cutting

blades with the dentin wall, where only two points of the rectangular cross section are

responsible for cutting.

Figure 6 – ProTaper NextTM

rectangular cross section (Dentsply Tulsa Dental).

This system is composed by five files, X1,X2, X3, X4 and X5, all in different

lengths (21,25 and 31 mm). In sequence, yellow, red, blue, double black and double

yellow identification rings corresponds to sizes 17/04, 25/06, 30/07, 40/06 and 50/06

respectively (Pérez-Higueras et al. 2015; Dentsply Maillefer).

Figure 7 – ProTaper NextTM

X1, X2, X3, X4 and X5 instruments (Dentsply Tulsa

Dental).



2014/2015


3. Protaper GoldRM

ProTaper GoldRM

(PTG), (Dentsply Tulsa Dental Specialties) instruments were

introduced recently in the US market. These files have a design that features identical

geometries as ProTaper® Universal, as well as the same instruments set and

manufacturer’s instructions for usage. Still, there is a differentiating heat-treatment with

the newest CM-Wire® technology (Hiewy et al. 2015; Uygun et al. 2015).

The full set of files is represent in Figure 8 and it is composed, as well, by two

shaping files, S1 and S2 and 5 finishing files, F1,F2,F3, F4 and F5.

Figure 8 – ProTaper GoldRM

system composed by shaping and finishing files (Dentsply

Tulsa Dental Specialties).

However, this system shows a different size of the handle, having eleven

millimeters compared to the thirteen from ProTaper® Universal system. According to

the manufacturer, this smaller handle allows improved accessibility to the tooth.

Hence, a different phase transformation behavior determines advanced

metallurgical and mechanical, resulting in improved flexibility and fatigue life. Yet, the

relationship between thermal behavior and fatigue properties of new PTG endodontic

instruments has not been investigated (Hiewy et al. 2015; Uygun et al. 2015).



2014/2015


II. Aims

The main aim of this in vitro study is to analyze the Fatigue Life of the recently

developed ProTaper GoldRM

NiTi instruments since little independent research is

available.

Beyond that, once manufacturers proclaim that this system has improved

flexibility and higher cyclic fatigue over ProTaper® Universal, other purpose of this

study is to examine the fatigue life of PTG system and compare it with its predecessor

and other ProTaper® in order to take further clinical decisions.

A. Specific goals

- To compare the fatigue life of instruments F2 and F3 of ProTaper

GoldRM

system.

H0 – The number of cycles until break is alike in both instruments.

H1 – The number of cycles until break is different in both instruments.

- To compare the length of fracture in instruments F2 and F3 of ProTaper

GoldRM

system.

H0 – The length of fracture is alike in both instruments.

H1 – The length of fracture is different in both instruments.

- To compare the fatigue life of F2 instruments of ProTaper GoldRM

with

F2 instruments of ProTaper® Universal.

H0 –Time to fracture is alike in both instruments.

H1 – Time to fracture is higher for PTG.

H2 –Time to fracture is higher for PTU.


with

F3 instruments of ProTaper® Universal.

H0 –Time until break is alike in both instruments.

H1 – Time until break is higher for PTG.

H2 –Time until break is higher for PTU.



2014/2015



with

X2 instruments of ProTaper NextTM

.

H0 –Time to fracture is alike in both instruments

H1 - Time to fracture is higher for PTG F2.

H2 –Time to fracture is higher for PTN X2.


with

X3 instruments of ProTaper NextTM

.

H0 –Time to fracture is alike in both instruments.

H1 - Time to fracture is higher for PTG F3.

H2 – Time to fracture is higher for PTN X3.

B. Main Goals

Through a bibliographic review, to compare the fatigue life of ProTaper GoldRM

instruments data with other studies.



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III. Materials and Methods

A. Instruments

For this in vitro study two types of variable taper rotary files from ProTaper

GoldRM

and ProTaper® Universal systems were used.

Those constituted four experimental groups, as seen in Table 1.

Group Type of

file n

Length

(mm) Taper

Experimental

Groups

1 PTG F2 12 25 0,08

2 PTG F3 12 25 0,09

3 PTU F2 12 25 0,08

4 PTU F3 12 25 0,09

Table 1– Samples distributed through the experimental groups with respective length (mm)

and taper.

Group 1 and group 3 had red identification rings on their handles corresponding

to D0 diameters of 0.25 mm, fixed tapers between D1 and D3 of 0,08 and “decreasing”

tapers from D14-D14. Group 2 and group 4 had blue identification rings with D0

diameter of 0,30 mm, fixed tapers between D1 and D3 of 0,09, and “decreasing” tapers

from D14-D14 as well (Ruddle 2008).

The samples used during experimental fatigue testes were sterilized and new,

without any previous utilization. Manufacturer DENTSPLY had no influence in the

present study (Fig. 9).

Figure 9 – Samples used for the in vitro study.



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B. Equipment

In order to carry out the fatigue tests, a mechanical system was developed by

Alexandre and Pinto in 2013 through a partnership between the Endodontics

Department of Faculdade de Medicina Dentária da Universidade de Lisboa and the

Mechanical and Industrial Engineering Department of Faculdade de Ciências e

Tecnologias da Universidade de Lisboa (Pinto 2013). The drawings, dimensions and

prototype can be seen in Figure 10.

Figure 10 - (a): Test bench with general measures (Pinto 2013). (b): System prototype

(Fernandes 2013).

This system was constituted by 2 pieces, 1 and 2, which articulate in order to

mimic one vertical root canal with a curvature angle of 45º and a radius of curvature

equal to 4, 7 mm.

Piece number 1 was manufactured by a Computerized Numerical Control

machine (CNC). Piece number 2 was manufactured from a rod of stainless steel

machined and hole-drilled. The stand structure was manufactured from a stainless steel

plate with 1,5mm thick with several folding, cutting and welding.

A point with specific coordinates was set: (4,026; 9,026) (Fig. 11). This point

establishes the place where the tip of the instrument should be in each test and had a

distance of 5 mm from the beginning of the curvature of the simulated root canal.



2014/2015


The configuration of the device and the location of W point are represented in

Figure 9. The instrument enters the mechanical system in (a), it’s forced to bend and

adjust to the curvature in (b) and its tip is visible in (c).

Figure 11 - Schematic representation of the mechanical system adapted from Pinto 2013.

As seen in Figure 12 (a), three bolts prevented the different pieces to move apart

so all system was static with the exception of the instrument tested. A malleable screen

of Teflon supported the device, which was fixed with two staples.

Figure 12 – (a): Mechanical System with the three bolts used to prevent the different

pieces to move apart and malleable screen of Teflon supporting the device. (b): PTG F3 during

CF testing. (c): PTU F3 during cyclic fatigue testing.



2014/2015


The motor used was a WaveOneTM

(Dentsply Maillefer), set at the ProTaper

Universal programme, with 300 rpm of continuous rotary motion and a torque of 4 N.

cm, following manufacturer’s recommendations.

One visible difference between PTG and PTU is the different size of the handle,

being smaller - 2 millimeters less - in this new system. Therefore, some modifications

were needed and made in the support structure, in order to adjust the tip of the file to the

W point. With a drilling engine, the original holes were extended, allowing the PTG file

to adjust, not falling short in the simulated root canal (Fig. 13).

Figure 13 – Support structure with extended holes.

All parameters guaranteed equal experimental conditions ensuring

reproducibility of the experiment.

C. Experimental procedure

The same methodology was used to test all instruments, on which the same

operator was responsible for the fulfillment of required steps. The procedure

comprehended the following steps:

1) Place the motor in the fixed system;

2) Place the instrument to be tested in the contra-angle and rotate the head

of the contra-angle until the instrument is parallel to the bench;

3) Make sure that the instrument is between pieces no. 1 and 2;



2014/2015


4) Adjust the instrument ensuring that it’s perpendicular to the upper part of

the block, the instrument is well adjusted between the two pieces and the

extremity of the file is well positioned at the W point (Fig. 12);

5) Tighten the three bolts and nuts according to the previous adjustments;

6) Turn on the WaveOneTM motor equipment and select the ProTaper

Universal programme;

7) Get the chronometer set up and ready to be use;

8) Step on the pedal initiating the chronometer at the same time, until

separation of the instrument occurs;

9) Stop the chronometer when the tip of the instrument comes off;

10) Remove the instrument off the contra-angle and measure the length of

the instrument with a ruler;

11) Repeat every step for all instruments.

The time each file took until fracture (t), was registered with a digital

chronometer. Time started at the beginning of the test and stopped at the moment the

operator detected instrument separation by observing and/or hearing the displacement of

the tip protruding from the artificial canal.

Since rotational speed employed in the fatigue test device was 300 rpm, the NCF

was determined by the following formula:

NCF = 300

0 5 t, in seconds

The fracture point in relation to the tip of the instrument was measured for each

experiment.

D. Statistical analysis

IBM® SPSS

® Statistics version 22.0.0 Software was used to carry out the

statistical analysis, on which initially a descriptive analyses was performed. For each

experimental group the mean, the standard deviation and the variance were calculated.



2014/2015


Subsequently, the Kolmogorov-Smirnov test was used to evaluate the data

obtained on time to fracture (sec), fracture length (mm) and Number of Cycles to

Fracture (NCF) for normal distribution.

Time to fracture and NCF revealed a normal distribution for all groups (K-S>0,

05). Thus, the analysis through the t-student test for independent samples was used in

order to clarify the correct hypotheses.

On the other hand, data for length of fracture was statically analyzed by a non-

parametric test, U Mann-Whitney for independent samples, since the data had no

normality for the distribution of group 1 and group 4 (K-S<0, 05).

Beyond that, data for mean time to fracture for group 1 and group 2 was

compared with data of ProTaper NextTM

X2 and X3 present in Vaz 2014 study,

respectively. Mean time to fracture between group 3 and X2 was also analyzed.

Since X2 data showed no normality (K-S<0,05), the U Mann-Whitney for

independent samples was used to analyze the mean times to fracture of group 1 and

group 3 with PTN X2. In the other hand, X3 data showed a normal distribution, so the

t-student test for independent samples was applied in order to compare the mean times

to fracture of group 2 with PTN X3.

The significance was set at 95% confidence level and differences were

considered statistically significant when p<0, 05.



2014/2015


IV. Results

Data obtained during the experimental tests regarding time to fracture, fracture

length and NCF for each type of file are presented in Table 2 (group 1 and group 2) and

Table 3 (group 3 and group 4).

Group Type of file Time to

fracture (sec)

Fracture length

(mm) NCF

1

F21 102,17 6,5 510,85

F22 114,07 6 570,35

F23 127,21 6,5 636,05

F24 52,5 8 262,5

F25 112,91 6,5 564,55

F26 113,01 6,5 565,05

F27 107,09 5 535,45

F28 139,28 6,5 696,4

F29 108,61 6,5 543,05

F210 91,34 6,5 456,7

F211 109,28 6 546,4

F212 140,32 7 701,6

2

F31 20,01 6,5 102,5

F32 42,4 6,5 210,2

F33 70,85 6 354,25

F34 52,87 7 264,35

F35 41,19 7 205,95

F36 82,35 7 411,75

F37 70,11 6,5 350,55

F38 64,63 6,5 323,15

F39 57,29 6 286,45

F310 66,79 6 333,95

F311 78,39 6,5 391,95

F312 59,7 6,5 298,5

Table 2 – Time to fracture, fracture length and NCF data from group 1 and 2 –

ProTaper GoldRM

instruments.



2014/2015


Group Type of file Time to

fracture (sec)

Fracture length

(mm) NCF

3

F21 57,37 5 286,85

F22 62,8 5,5 314

F23 55,63 6,5 278,15

F24 63 7 315

F25 39,06 6,5 195,3

F26 61,21 7 306,05

F27 58,56 7 292,8

F28 60,1 7 300,5

F29 61,21 7 306,05

F210 58,56 6 245,8

F211 60,01 5,5 286,45

F212 54,97 7 274,85

4

F31 34,98 6 174,9

F32 35,61 6 178,05

F33 25,37 6,5 126,85

F34 24,59 6,5 122,95

F35 32,73 6 163,65

F36 27,51 6,5 137,55

F37 34,52 6 172,6

F38 25,86 6 129,3

F39 25,27 6 126,35

F310 34,25 6 171,25

F311 28,34 6 141,7

F312 51,31 5,25 256,55

Table 3 – Time to fracture, fracture length and NCF data from group 3 and 4 –

ProTaper® Universal instruments.

Descriptive statistics for the different experimental groups over time to fracture,

fracture length and NCF are displayed in Table 4.



2014/2015


Group

Type of

file Quantity

Mean ± St.

Deviation Variance

Time to

fracture

(sec)

1 PTG F2 12 109,82 ± 23,03 486,10

2 PTG F3 12 58,18 ± 17,01 311,93

3 PTU F2 12 56,70 ± 6,78 45,96

4 PTU F3 12 31,70 ± 7,51 56,46

Length of

fracture

(mm)

1 PTG F2 12 6,46 ± 0,70 0,48

2 PTG F3 12 6,5 ± 0,37 0,14

3 PTU F2 12 6,42 ± 0,73 0,54

4 PTU F3 12 6,06 ± 0,34 0,12

NCF

1 PTG F2 12 549,08 ± 115,14 13257,19

2 PTG F3 12 294,46 ± 87,97 7749,38

3 PTU F2 12 283,48 ± 33,90 1148,99

4 PTU F3 12 158,48 ± 37,57 1411,44

Table 4 – Descriptive analysis: mean, standard deviation and variance regarding time,

length of fracture and NCF.

An overview of the mean NCF for each group is present in Graphic 1 as well.

Graphic 1 – Graphical representation of NCF for group 1, 2, 3 and 4.

0

100

200

300

400

500

600

Group 1 Group 2 Group 3 Group 4

Number of Cycles to Failure (NCF)

among Experimental Groups

NCF



2014/2015


In order to compare ProTaper GoldRM

and ProTaper NextTM

systems, the data

regarding PTN X2 and PTN X3 instruments for time to fracture from Vaz 2014 study

was selected, since the corresponding instruments PTG F2/PTN X2 and PTG F3/PTN

X3 are used for similar purposes during treatment (Dentsply Tulsa Dental brochure).

Type of

file

Quantity Mean ± St.

Deviation

Variance

Time to fracture (sec) PTG F2 12 109,82 ± 23,03 486,10

PTN X2 16 77,8 + 9,3 87,4

PTG F3 12 58,18 ± 17,01 311,93

PTN X3 4 89,3 + 9,5 89, 4

Table 5 – Mean time to fracture data from the present study and from Vaz 2014 study.

Through statistical analysis previously described and considering the initially

formulated hypothesis, the following data were earned:

- The mean value of NCF between group 1 (549, 08 ± 115, 14) and group

2 (294, 46 ± 87, 97) was found to have a significant statistical difference, rejecting the

null hypothesis (H0) (p <0.001).

- There was not found a significant statist difference concerning length of

fracture among group 1 (6, 46 ± 0, 70) and group 2 (6, 5 ± 0, 37), retaining the null

hypothesis (H0).

- The mean value of time to fracture between group 1 (109, 82 ± 23, 03)

and group 3 (56, 70 ± 6, 80) was found to have a significant statistical difference,

rejecting the null hypothesis (H0) (p <0.001).

- The mean value of time to fracture between group 2 (58, 18 ± 17, 01) and

group 4 (31, 70 ± 7, 51) was found to have a significant statistical difference, rejecting

the null hypothesis (H0) (p <0.001).

- The mean value of time to fracture between group 1 (109, 82 ± 23, 03)

and PTN X2 (77, 8 + 9, 3) was found to have a significant statistical difference,

rejecting the null hypothesis (H0) (p <0.001).

- The mean value of time to fracture between group 2 (58, 18 ± 17, 01) and

PTN X2 (77, 8 + 9, 3) was found to have a significant statistical difference, rejecting

the null hypothesis H0 (p=0,006).



2014/2015


Beyond that, statistic analyses show:

- The mean value of NCF between group 3 (283, 48 ± 33, 90) and group 4

(158, 48 ± 37, 57) was found to have a significant statistical difference (p <0.001).

- There was not found a significant statistical difference for mean time to

fracture within group 2 (58, 18 ± 17, 01) and group 3 (56, 70 ± 6, 80).

- A significant statistical difference, concerning the mean values for length

of fracture among group 2 (6, 5 ± 0, 37) and group 4 (6, 06 ± 0, 34) was found (p = 0,

014).

A graphical demonstration shows a visible difference among 3 groups of

ProTaper® Universal, ProTaper Next

TM and Protaper Gold

RM systems analyzed (Grap.

2).

Graphic 2 – Mean time to fracture (sec) for group 1 – PTG F2, PTN X2 and group 3 –

PTU F2.

0 50 100 150

Group 1

PTN X2

Group 3

Time to Fracture (t) for group 3,

PTN X2 and group 1

t (sec)



2014/2015


V. Discussion

In many cases, fracture of rotary NiTi instrument occurs because of incorrect or

excessive use, which stresses the importance of correct training in the use of rotary NiTi

technology. However, many factors have been linked to the propensity for fracture of

rotary NiTi instruments (Parashos & Messer 2006).

The main aim for this in vitro study is to analyze the fatigue life of the new

Protaper GoldRM

system. Moreover, since manufacturer proclaims that this system has

improved flexibility and higher resistance to cyclic fatigue over ProTaper® Universal

system, other purpose of this study is to compare it with its predecessor in order to take

further clinical decisions.

Time to failure data (t) was recorded along the experimental procedure and NCF

was determined afterwards. These two parameters have been used to assess cyclic

fatigue resistance over time, in which t presents more clinically relevance information,

as time is much easier for the operator to observe than the number of cycles the

instrument endures. In addition, NCF offers more pertinent information regarding the

ability of the instrument design to withstand cyclic fatigue (Wan et al. 2011).

- PTG F2 proved to be significantly more resistant to cyclic fatigue than

PTG F3 with higher mean of NCF and time to fracture. The same trend was verified

when evaluating the relation among PTU F2 and PTU F3. These findings can be easily

explained since resistance of rotary instruments to cyclic fatigue decreases when

instrument sizes and respective diameter increases, on instruments of the same design (

Fife et al. 2004; Ullman & Peters 2005; Wollcot et al. 2006; Plotino et al. 2009; Sheng

et al. 2010; Pérez-Higueras et al. 2014; Capar et al. 2015).

- A significant statistical difference concerning length of fracture among

PTG F2 (6,46 ± 0,70) and PTG F3 (6,5 ± 0,37) was not shown; the same happened for

PTG F2 (6,46 ± 0,70) and PTU F2 (6,42 ± 0,73). However, significant statistical

difference between PTG F3 (6, 50 ± 0, 40) and PTU F3 (6, 06 ± 0, 34) was found.

In previous studies, it has been reported that fracture occurred usually at the

point of maximum flexure (Pruett et al. 1997; Plotino et al. 2009). The point of

maximum flexure was at 5 mm from the tip in this experiment. The difference between

the mean lengths of fracture to this value may be due to an inaccurate measurement



2014/2015


since a ruler was used. The use of an X-Y coordinates measuring table, for example,

would be more accurate.

Nevertheless, recently Capar et al. showed that different instruments subjected

to the same cyclic fatigue testing setup (same working length) fractured at different

working lengths. The researchers attributed their results to differences in the bending

moments of the instruments, which were manufactured from different alloys (Capar et

al. 2015). Furthermore, a system designed to simulate a root canal that doesn’t constrain

a precise trajectory may alter bending properties of different files, even if they have the

same dimensions (Plotino et al. 2009). A previous study has shown that if the artificial

canal does not sufficiently restrict the instrument shaft, it would tend to spring back into

its original straight shape, aligning into a trajectory of greater radius and reduced angle

(Larsen et al., 2009). Gutmann & Gao 2012 discussed this limitation of cyclic fatigue

testing in steel canals as well.

- PTG F2 and F3 proved to be significantly more resistant to cyclic fatigue

than PTU F2 and F3, respectively. Despite the identical architecture and operation of

the PTG and PTU systems, the different manufacturing processes of the instruments

clearly affect their fatigue resistance behaviors. Several authors state that a higher

proportion of martensite (which is known to be more flexible than austenitic NiTi) and

changes in the phase transformation behavior may be the reason (Hayashi et al. 2007;

Shen et al. 2011; Hieawy et al. 2015; Uygun et al. 2015). Moreover, CM-Wire® was

proven to be significantly more resistant to fatigue than instruments produced using

traditional NiTi (Plotino et al. 2012; Shen et al. 2012).

Data obtained from Vaz 2014 during in vitro studies concerning cyclic resistance

of ProTaper NextTM

system was also used to compare the variable time to fracture

among PTG F2 / PTN X2 and PTG F3 / PTN X3 instruments. This particular study was

underlined under exactly the same conditions, with the same experimental assembly and

procedure, which decreased the number of variables, ergo less bias. Regarding this

comparison:

- The mean time to fracture for PTG F2 (109, 82 ± 23, 03) was

significantly higher than time to fracture for PTN X2 (77, 8 ± 9, 3). . Beyond that, time

to fracture for PTN X2 was significantly higher than time to fracture for PTU F2 (56,70

± 6,78) (p<0,001).These results are in agreement with Uygun et al. study that showed

higher values for cyclic fatigue for PTG instruments.



2014/2015


Different characteristics could be responsible for these findings; for example a

different NiTi alloy composition and different cross section’s geometry. PTG are

manufactured with CM-Wire® technology and triangular cross section’s; PTN of M-

Wire®, with off-centered rectangular cross-section (Pérez-Higueras et al. 2014; Uygun

et al. 2015). Plotino et al. stated that within instruments produced using CM-Wire®, a

higher cyclic fatigue was present when comparing with M-wire®

and conventional

alloys as previously mentioned (Plotino et al. 2012).

Moreover, it has been shown that cross-sectional design has an impact on the

stress developed by an instrument under either tension or bending (Zhang et al. 2010;

Pérez-Higueras et al. 2014). A triangular cross-sectional design, present in PTG and

PTU systems, was showed to possess a higher cyclic fatigue resistance than a square

cross-sectional design (Cheung et al. 2011). This difference is related to the reduced

metal mass of the instruments with a triangular cross-section compared with instruments

with a square cross section of a similar diameter (Wu & Wesselink 1993; Capar et al.

2015; Uygun et al. 2015).

- Time to fracture of PTG F3 (58, 18 ± 17, 01) was significantly lower

than time to fracture for PTN X3 (89, 3 ± 9, 5). Yet, different mean lengths of fracture

were comprised between the two types of instruments. Fracture length data of X3 (4.5 ±

0.5) was much lower in relation to PTG F3 data (6.5 ± 0.37). That way, the local where

the fracture occurred for PTN F3 had a smaller diameter. As mentioned above in the

text, a larger diameter at the point of fracture will lead to a shorter time to fracture

which can explain the reduced cyclic fatigue observed for PTG F3.

On the other hand, Uygan et al. noted that despite PTG had higher values for

cyclic fatigue at all levels when comparing with PTN, when cyclic fatigue tests were

performed at a distance equal to 8 mm from the tip, a point were the diameter between

both instruments is similar, the difference was not significant.

That being noted, and taking to account that experimental data obtained with

PTN X3 group in Vaz 2014 involved a small sample (n = 4), a possible conclusion is

compromised.

To add, in order to compare data gathered in this study with current literature on

the same subject, Table 6 summarize the type of instruments, testing conditions such as

rotational speed and respective results on NCF and time to fracture.



2014/2015


Article Type of

instrument

Testing

conditions

Rotacional

speed

(rpm)

NCF

(mean/st.dev)

Time to

Fracture

(mean/st.dev)

Protaper

GoldRM

present

study

F2

4,7 mm of

radius

45˚

Dry

conditions

300

549,08 ±

115,14

109,82 ±

23,03

F3 294,46 ±

87,97 58,18 ± 17,01

Uygun

et al.,

2015

F2 (5 mm

from the

tip)

3 mm of

radius

0˚

Oil of

lubrification

300 --

12219 67 ±

2089 44

F2 (8 mm

from the

tip)

3904 50 ±

520 63

Hiewy

et al.,

2015

F2

6 mm of

radius

40˚

Deionized

water

300

985,2 ± 135,5

--

F3 835,5 ± 119,3

Table 6 – Summary conditions, design and results of two studies made for PTG cyclic

fatigue testing.

Even within the same file system major differences can be noticed respecting

NCF and time to fracture data among different studies.

The variable results can be attributed to significant details that differ among the

experimental methods. Some of these factors include:

A. Radius of curvature

An increase on radius of curvature was proven to decrease time to fracture

(Tripi et al. 2006; Inan et al. 2007; Kim et al. 2010).

Uygun et al. used a lower value of radius, which, according to literature, could

lead to a higher time to fracture. On the other hand, Hiewy et al. had a higher value in



2014/2015


this parameter and still the NCF values were higher. Thus, other variables must be taken

to account.

B. Angle of curvature

Several studies showed that an increase in angle of curvature was found to

decrease fracture time (Ullman & Peters 2005; JR et al. 2007; Wan et al. 2011).

In both studies used for this comparison, this value was higher.

C. Lubrification

Pilot experiments had indicated that lubrication with various agents did not

result directly in different cyclic fatigue scores but helped to reduce heat generated,

leading to a higher fatigue life (Ullman & Peters 2005; Shen et al. 2012). Moreover,

Shen et al. showed that cyclic fatigue of CM-Wire® instruments is longer in liquid

media than in air.

Taking this into account, a plausible explanation for such lower values in the

present study may be due to the dry conditions used, with absence of lubrification along

the tests carried out. In Uygun et al. and Hieawy et al., a lubrificating oil or liquid

media was used.

Moreover, environmental conditions have shown to significantly affect the

fatigue behavior of NiTi rotary instruments and fatigue tests should be carried out in

similar environmental conditions, as suggested by Plotino et al. and Shen et al.

Some more limitations can be noticed in the present study as far it concerns

testing cyclic fatigue of rotary instruments.

For example, to date, there is no specification or international standard to test

cyclic fatigue resistance of endodontic rotary instruments. Such a new standard is

required in order to minimize uncontrolled variables, and to define suitable mechanical

properties of NiTi rotary instruments for a safe, efficient clinical use and to introduce

universally accepted testing devices for experimental evaluation of products or

prototypes. In addition, a consensus between researchers should also be reached to find

the most accurate statistical analysis (Plotino et al. 2009).

It has been also reported that static cyclic fatigue tests (with no axial movement)

showed lower results when compared with dynamic tests in which endodontic



2014/2015


instruments are subjected to axial movements, even though it minimizes the effect of

variables. Moreover, the instrument is generally tested beyond the time that the

instrument is expected to be active at a specific level when shaping a root canal

normally in the clinic. Therefore, higher cyclic fatigue resistance is expected in a

clinical situation in which instruments are operated in a constant in and out motion that

helps to avoid taper lock (Pérez-Higueras et al. 2014).

Beyond that, stainless steel canals do not exhibit similar properties to those of

root canals found in real teeth. It is possible that instruments may perform in a different

manner when used clinically (Wan et al. 2011).



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VI. Conclusion

Some challenges can be found during root canal preparation, like complex

anatomy of the root canal system and instrumental limitations inherent to treatment.

An increasing concern about instrument fracture during clinical procedure urges

since NiTi rotary instruments have been gaining popularity in endodontic treatment.

Several authors advocate that the great majority of instruments appeared to have failed

because of cyclic fatigue. Therefore, understanding cyclic fatigue of different

endodontic instruments may be a subject of interest.

The purpose of this study was to characterize the cyclic fatigue of Protaper

GoldRM

instruments, and compare it to other rotary systems.

Regarding Protaper GoldRM

system, F2 instrument showed superior cyclic

fatigue resistance when compared with F3. More, comparing data from PTG F2 and F3

with ProTaper® Universal F2 and F3, respectively, ProTaper Gold

RM showed a superior

behavior on cyclic fatigue resistance, with higher time to fracture (PTG F2 > PTGF3 ≥

PTU F2 > PTU F3).

When comparing data from this study with and analogue that undergone the

same testing conditions and assembly line from ProTaper NextTM

system, PTG F2

instrument proved to have a higher time to fracture over PTN X2, hence a better cyclic

fatigue performance (PTG F2 > PTN X2 > PTU F2).

To date, there’s no specification or international standard to test cyclic fatigue

resistance of endodontic rotary instruments. Thus, different results may arise.

That being stated, it is important for clinicians to understand the differences

between systems of files to take advantage of the latest technology and facilitate good

choices to meet anatomic challenges.



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Fátima Pereira ix FMDUL

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APPENDIX

Abbreviations

CNC - Computerized Numerical Control

CM - Control Memory

D - Diameter

M - Memory

NCF - Number of Cycles to Fracture

NiTi - Nickel-Titanium

PTG - ProTaper Gold

PTN - ProTaper Next

PTU - ProTaper Universal

TTR - Transformation Temperature Range

K-S – Kolmogorov-Smirnov

Symbols

% - percentage

n - number of sample

p - significance

® - registered trademark

RM – reference model

TM - unregistered trademark

Units

º - degrees

ºC - degree Celsius

sec - seconds

mm - millimeters

N. cm - newton centimeter

rpm – rotations per minute

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